This invention relates to the bonding of shaped members and, more particularly, to the bonding of shaped members without the application of an adhesive between or to either of the surfaces to be bonded.
The method of the present invention is especially well suited for use in bonding shaped members in the form of sheets or films, one of which comprises a thermoplastic resin such as polyethylene or ethylene-vinyl acetate copolymer (EVA). To avoid ambiguity, the process by which two members, for example, extended sheets or films are bonded together will be termed "lamination" and the product formed thereby will be referred to as a "laminate". When a laminate is formed in accordance with the lamination process of the present invention, the bond is effected without a third substance -- that is to say, no adhesive is used -- and such a lamination process is herein referred to as "dry lamination".
In accordance with conventional lamination practice, an adhesive is applied to one or both of the members to be bonded together. To be commercially desirable, the adhesive utilized should be effective in thin layers; it should be colorless and transparent; and it should be indefinitely stable and resistant to extremes of temperature. Such processes and the products produced thereby leave much to be desired. Adhesives which embody the desired properties are relatively expensive, usually costing substantially more than the materials to be bonded. The adhesive bond strength is not readily, if at all, controllable and when a bond is required which is to be separated at an appropriate time, what often results are laminates that either cannot be separated without destroying them or the bond fails prematurely.
U.S. Pat. No. 3,188,265 relates to a known dry lamination process for making polyethylene-polyester laminates. Instead of an adhesive such as nitrocellulose, the process of that patent utilizes ultraviolet radiation to bond a layer of polyethylene which has been extruded onto polyester stock. It is believed that the ultraviolet radiation is effective in that system because of a unique combination of an ultraviolet radiation-transparent layer (polyethylene) on an ultraviolet radiation opaque layer (polyester). The UV radiation passes through the polyethylene layer but is substantially completely attenuated when it reaches the polyester layer, thereby releasing all of its energy in an interface region of molecular thickness. The result is a hardened, or cross-linked but transparent inner film a few molecular layers thick which serves to bond the polyethylene and polyester layers together. While the process of the U.S. Pat. No. 3,188,265 provides a laminate without requiring the presence of an adhesive it has certain drawbacks. High power UV lamps are not only expensive but their use requires that careful measures be taken to ensure the safety of personnel. Effective shielding is required. Ozone and nitrogen oxides must be removed from the area and equipment must be made of materials which are not rapidly degraded by the unwanted by-products of the radiation process, e.g. ozone and nitrogen oxides, when it is carried out in air.
From U.S. Pat. No. 3,018,189 it is known to expose a polyethylene body to an electrostatic or corona discharge to improve the adherence of printing inks and other such coating materials to the surface of the polyethylene body. That patent also states that to render the plastic films [polyethylene] or laminae receptive to the adhesive, each of the films to be bonded is first exposed to the corona discharge and then adhesive is applied to one or both of the treated surfaces to be joined. The thus treated films or laminae are then to be brought into contact with each other, with the adhesive and treated sides together.
U.S. Pat. No. 3,360,412 relates to a dry lamination process wherein at least one of the surfaces to be bonded is a thermoplastic film which is exposed to an open flame or electrical discharge and then subsequently is brought into contact with a dissimilar material while the latter is at least at a temperature equal to the fusion temperature of the thermoplastic film. A drawback of this process is that the thermoplastic material is heated to or above its fusion temperature so that it is at least in a semifluid condition, thereby making the material likely to curl or shrivel.
U.S. Pat. No. 3,823,061 describes a dry lamination process wherein the films to be bonded are treated with an electrical corona discharge. The patent relates to a corona treatment in ambient air which is accomplished by passing the laminae between an electrode pair or an electrode and a roller at some time before the laminae are compressed together.